How to make the method getItem from the offlineStorage of the Kendo Datasource work with LocalForage

I need to setup my datasource to use localForage to manage my offline data storage.

The problem I'm having is that localForage is asynchronous by nature, only allowing us to use a callback or a promise to retrieve data.

This is how my code is set-up:

(function () {
    'use strict';

    var serviceId = 'employeeDataSource';
    angular.module('app').factory(serviceId, function () {
        var crudServiceBaseUrl = 'Data/Employees';
        var dataSource = new kendo.data.DataSource({
            type: "odata",
            offlineStorage: {              
                getItem: function () {
                    localforage.getItem('employees-key').then(function (value) {
                        console.log(value);                                               
                        return JSON.parse(value);
                    });                   
                },
                setItem: function (item) {
                    if (item.length > 0) {
                        localforage.setItem("employees-key", JSON.stringify(item));
                    }                   
                }
            },
            transport: {
                read: {
                    url: crudServiceBaseUrl + "/",
                    dataType: "json"
                },                               
                update: {
                    url: function (data) {
                        return crudServiceBaseUrl + "(guid'" + data.EmployeeId + "')";
                    }
                },
                create: {
                    url: crudServiceBaseUrl
                },
                destroy: {
                    url: function (data) {
                        return crudServiceBaseUrl + "(guid'" + data.EmployeeId + "')";
                    }
                }
            },
            batch: false,
            pageSize: 5,
            serverPaging: true,
            schema: {
                data: function (data) {
                    return data.value;
                },
                total: function (data) {
                    return data['odata.count'];
                },
                model: {
                    id: "EmployeeId",
                    fields: {
                        EmployeeId: { editable: false, nullable: true },
                        Name: { validation: { required: true } },
                        Email: { validation: { required: true } },
                        IsManager: { type: "boolean" },
                        MaxHolidaysPerYear: { editable: false, type: "number", validation: { min: 0, required: true } },
                        HolidaysLeftThisYear: { type: "number", validation: { min: 0, required: true } }
                    }
                }
            },
            error: function (e) {
                if (e.xhr.responseText !== undefined) {
                    console.log(e.xhr.responseText);
                }
            }
        });
        dataSource.online(navigator.onLine);
        $(window).on("offline", function () {
            dataSource.online(false);
        });
        $(window).on("online", function () {
            dataSource.online(true);
        });
        return dataSource;
    });
})();

When off-line, the getItem gets called, then the setItem gets call as well with an empty array, hence the:

if (item.length > 0) {
        localforage.setItem("employees-key", JSON.stringify(item));
}

When the promise finally returns the off-line data (with the correct values I expected), the Grid displays no results.

This behaviour is presumably because of the promise ?

I tried the same thing with sessionStorage and its worked perfectly... i.e.:

getItem: function () {
     return JSON.parse(sessionStorage.getItem("employees-key"));
},
setItem: function (item) {
     sessionStorage.setItem("employees-key", JSON.stringify(item));
}

What can I do to get around this?

Just got a heads-up from Telerik

There is an issue logged in their GitHub repo about the same problem.

You can keep track on the progress here:

https://github.com/telerik/kendo-ui-core/issues/326